This invention relates to a polymer of essentially balanced charge which is obtained from the polymerization of a cationic-anionic monomer pair and a neutral comonomer and to a hydrophilic contact lens fabricated from the polymer.
Hydrophilic contact lenses are generally formed from covalently crosslinked polymers which are based on hydrophilic derivatives of acrylic or methacrylic acid, e.g., their hydrophilic esters or amides, hydrophilic vinylic polymers such as vinylpyrrolidone, and the like. In their hydrated state, these polymers are referred to as hydrogels, coherent three-dimensional polymer structures or net-works which are capable of absorbing large quantities of water without dissolving and of transporting oxygen. In addition to the hydrophilic monomer(s), the preparation of hydrogels used in the manufacture of contact lenses also utilizes minor amounts of less hydrophilic, and even hydrophobic, monomer(s) to confer mechanical strength and other useful properties.
One of the troublesome aspects of known types of hydrogel contact lenses lies in their tendency to accumulate various soils contained in tears, e.g., proteins, lipids, mucoids, etc., on their surfaces. The explanation for this tendency is not all that certain but is believed to be due to the interaction of the charged groups present on the soil components and the charge associated with the hydrogel. For instance, it is known that lysozyme, an enzyme present in tears, possesses a net positive charge and forms complexes with polymers such as the known types of hydrogels which possess a net negative charge This interaction of hydrogel polymer surface and soil represents a significant disadvantage for contact lenses manufactured therefrom.
In an effort to counteract this soil accumulation tendency, U.S. Pat. No. 4,569,858 proposes two methods for treating a lens manufactured from an unhydrated polymer having reactive groups, e.g., hydroxyl and/or acid groups, on its surface with a compound which reacts with such groups to provide a soil deposition-resistance hydrophobic coating, hydrophilic coating or charged surface. In the first of the two methods, the surface of the unhydrated lens is pretreated with a dilute solution of a strong organic base in a nonswelling solvent, e.g., potassium tert-butoxide in ether, to convert surface hydroxyl groups to alkoxide groups followed by reacting alkoxide and carboxylate groups with an alkylating, acylating or sulfonating agent to provide a hydrophobic surface. In the second method, the surface hydroxyl and/or acid groups are directly reacted with a reagent such as a diazoalkane to provide a soil deposition-resistant surface.
In yet another approach to imparting soil deposition-resistance to a hydrogel contact lens, U.S. Pat. No. 4,650,843 discloses forming the lens from a monomeric composition containing 50-95 weight percent of a monomethacrylate of an alkylene glycol, 5-35 weight percent of at least one fluorine-containing monomer selected from the group consisting of trifluoroethyl methacrylate, hexafluorisopropyl methacrylate, pentafluoro-n-propyl methacrylate and pentafluoro-n-butyl methacrylate, and less than 40 weight percent of one or more compounds selected from the group consisting of an unsaturated carboxylic acid having one or more carboxyl groups in the molecule, a monomethacrylate of a polyhydric alcohol having three or more hydroxyl groups, and an alkyl methacrylate.
U.S. Pat. No. 4,163,609 describes a contact lens material, the surface of which is self-cleaning of organic debris, by changing the surface charge from a natural anionic positive charge which attracts debris to a cationic negative charge which repels debris.